1. Field of the Invention
The present invention relates to heavy duty pneumatic bias tires. More particularly it relates to an improvement in durability of such a heavy duty pneumatic bias tire wherein a carcass of a bias structure is composed of a plurality of carcass plies each containing organic fiber cords arranged at a cord angle with respect to the median equational plane of the tire, a plurality of bead rings are embedded in each of bead portions and the end portions of the carcass plies are turned-back around each of the bead rings from the inside to the outside of the tire.
2. Related Art Statement
Heretofore, in trucks, construction vehicles and the like there have been employed large heavy duty pneumatic bias tires having a large number of carcass plies in order to provide a necessary strength corresponding to the condition for using.
Such a conventional heavy duty pneumatic bias tire, for example as shown in FIG. 9, has a tread portion 1, a pair of sidewall portions 2, bead portions 3 at the radially inner ends of the sidewall portions 2 and a plurality of carcass plies 4 toroidally extended from the tread portion 1 to the sidewall portions 2 and the bead portions 3. The carcass plies 4 are layered with their organic fiber cords arranged at an angle to the median equational plane of the tire.
Particularly, in such a heavy duty pneumatic bias tire having twenty or more carcass plies, in order to securely retain the end portions of the carcass plies 4 in the bead portions 3 of the tire and to closely seat the bead portions against a wheel rim 5, a plurality of bead rings 6 are embedded in each of the bead portions 3 and spaced from each other in the axial direction. The end portions of a plurality of carcass plies are turned-back around each of the bead rings 6 from the inside to the outside of the tire.
The height of the turned-back ply ends 4a of the carcass plies 4 are determined by balance of the carcass rigidity. The turned-back ply ends 4a are usually terminated in the sidewall portions 2 of the tire at a different height such that the thickness of the carcass smoothly increases as it comes near the bead portion 3. The plurality of turned-back ply ends 4a are therefore spaced in the direction from the bead portion 3 to the sidewall portion 2 by a ply step S of usually 15-40 mm corresponding to the distance between the successive turned-back ply ends as shown in FIG. 9.
Thus, the heavy duty pneumatic bias tire having a large actual number of carcass plies and a thick case gauge, includes many turned-back ply ends of carcass plies which are turned-back around each of the bead rings in the bead portions 3 and the sidewall portions 2 of the tire.
These turned-back ply ends have been pointed out as weak points in tire structure since the exposed cut ends of organic fiber cords such as nylon cords directly contact the surrounding rubber having a rigidity different from that of the fiber cord.
Heretofore, a countermeasure for eliminating such a weak point is by covering the ply ends with a rubber sheet or increasing the gauge of rubber surrounding the ply ends.